The present invention relates generally to EPAS systems. More specifically, the present invention relates to rack and pinion configurations for optimizing EPAS systems.
The prior art provides various examples of assisted steering systems. These systems, no matter the mode of operation, serve to assist manual steering by a driver of a vehicle in order to facilitate steering of the vehicle. The most common type of assisted steering system utilizes hydraulics. These systems have several disadvantages that arise due to their use of hydraulic systems. For example, they require the use of a hydraulic pump that is typically connected to the engine. This arrangement can reduce engine power and fuel efficiency.
Electric power assisted steering (EPAS) systems include the use of an electric motor and various sensors. These systems assist manual steering by sensing the initiation of steering by a driver and assisting the steering through activation of the motor. The motor typically drives an assist pinion that facilitates movement of a rack. EPAS systems provide several advantages, such as the elimination of the hydraulics associated with other types of power steering.
EPAS systems, however, still have several drawbacks. For example, while these systems eliminate hydraulics, mechanical interactions between components are an integral part of the systems. Most conventional EPAS systems use a dual pinion configuration in which a driver pinion and an assist pinion interact with a rack to affect steering. The driver pinion is connected to the steering wheel while the assist pinion is connected to the electric motor. Both pinions mechanically engage the rack through interaction of gear sets on the rack and pinions. As with any gear set, an engagement pulse is produced each time a pinion tooth engages a rack tooth. Engagement pulses produce a jerking motion that can travel throughout the steering system due to the interconnection of all parts. Because the systems utilize two pinions, the engagement pulses, when overlapping, can amplify, increasing the jerking in the system.
The present invention provides an EPAS system with dual pinions that are placed out of phase relative to each other. That is, the EPAS system according to the present invention has an assist pinion that is out of phase with the driver pinion. When one pinion is fully engaged with one or more teeth of the rack, the other pinion is partially engaged with one or more teeth of the rack. As a result the amplification problem is avoided. Indeed, an engagement pulse of one pinion can be minimized due to the lack of an engagement pulse of the other pinion at a particular time. Consequently, in contrast to the additive effect of engagement pulses found in prior art systems, the engagement pulses associated with the EPAS system of the present invention can be minimized or canceled. This, in turn, minimizes or eliminates the jerking in the steering system.
In one preferred embodiment, the present invention provides an EPAS system comprising a rack having first and second sets of rack teeth, a driver pinion having a set of driver teeth, and an assist pinion having a set of assist teeth. The assist teeth are only partially engaged with one or more rack teeth while the driver teeth are fully engaged with one or more rack teeth.
In any given steering system, various parameters determine the details of the componentry utilized. For example, driver rack speed, number of driver pinion teeth, number of assist pinion teeth, and the assist rack speed all affect each other. The EPAS system of the present invention is produced by optimizing three of these parameters based on a given fourth parameter.
Accordingly, the present invention also provides a method of supplying an EPAS system to a customer. In a preferred embodiment, the method comprises obtaining a desired driver rack speed from the customer. This number is related to a desired steering ratio for a particular vehicle into which the steering system will be incorporated. Next, the method includes optimizing the number of driver teeth, the number of assist teeth, and the assist rack speed such that a driver pinion and an assist pinion are placed out of phase with each other.
While the invention is defined by the claims appended hereto, additional understanding of the invention can be obtained by referencing the following detailed description of preferred embodiments and the appended drawings.